Hypoxic pulmonary vasoconstriction: redox events in oxygen sensing

Redox signaling and reactive oxygen species in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an essential physiological mechanism of the lung that matches blood perfusion with alveolar ventilation to optimize gas exchange. Perturbations of HPV, as may occur in pneumonia or adult respiratory distress syndrome, can cause life-threatening hypoxemia. Despite intensive research for decades, the molecular mechanisms of HPV have not been fully elucidated. Reactive oxygen species (ROS) and changes in the cellular redox state are proposed to link O2 sensing and pulmonary arterial smooth muscle cell contraction underlying HPV. In this regard, mitochondria and NAD(P)H oxidases are discussed as sources of ROS. However, there is controversy whether ROS levels decrease or increase during hypoxia. With this background we summarize the current knowledge on the role of ROS and redox state in HPV.

Interactions between calcium and reactive oxygen species in pulmonary arterial smooth muscle responses to hypoxia

In contrast to the systemic vasculature, where hypoxia causes vasodilation, pulmonary arteries constrict in response to hypoxia. The mechanisms underlying this unique response have been the subject of investigation for over 50 years, and still remain a topic of great debate. Over the last 20 years, there has emerged a general consensus that both increases in intracellular calcium concentration and changes in reactive oxygen species (ROS) generation play key roles in the pulmonary vascular response to hypoxia. Controversy exists, however, regarding whether ROS increase or decrease during hypoxia, the source of ROS, and the mechanisms by which changes in ROS might impact intracellular calcium, and vice versa. This review will discuss the mechanisms regulating [Ca2+]i and ROS in PASMCs, and the interaction between ROS and Ca2+ signaling during exposure to acute hypoxia.

Apparent PKA activity responds to intermittent hypoxia in bone cells: a redox pathway?

We studied hypoxia-induced dynamic changes in the balance between PKA and PKA-counteracting phosphatases in the microfluidic environment in single cells using picosecond fluorescence spectroscopy and intramolecular fluorescence resonance energy transfer (FRET)-based sensors of PKA activity. First, we found that the apparent PKA activity in bone cells (MC3T3-E1 cells) and endothelial cells (bovine aortic endothelial cells) is rapidly and sensitively modulated by the level of O(2) in the media. When the O(2) concentration in the glucose-containing media was lowered due to O(2) consumption by the cells in the microfluidic chamber, the apparent PKA activity increases; the reoxygenation of cells under hypoxia leads to a rapid ( approximately 2 min) decrease of the apparent PKA activity. Second, lack of glucose in the media led to a lower apparent PKA activity and to a reversal of the response of the apparent PKA activity to hypoxia and reoxygenation. Third, the apparent PKA activity in cells under hypoxia was predominantly regulated via a cAMP-independent pathway since 1) changes in the cAMP level in the cells were not detected using a cAMP FRET sensor, 2) the decay of cAMP levels was too slow to account for the fast decrease in PKA activity levels in response to reoxygenation, and 3) the response of the apparent PKA activity due to hypoxia/reoxygenation was not affected by an adenylate cyclase inhibitor (MDL-12,330A) at 1 mM concentration. Fourth, the immediate onset of ROS accumulation in MC3T3-E1 cells subjected to hypoxia and the sensitivity of the apparent PKA activity to redox levels suggest that the apparent PKA activity change during hypoxia and reoxygenation in this study can be linked to a redox potential change in response to intermittent hypoxia through the regulation of activities of PKA-counteracting phosphatases such as protein phosphatase 1. Finally, our results suggest that the detection of PKA activity could be used to monitor responses of cells to hypoxia in real time.

Vasoactivity of hydrogen sulfide in normoxic and anoxic turtles (Trachemys scripta)

Systemic vascular resistance ( Rsys) of freshwater turtles increases substantially during anoxia, but the underlying mechanisms are not fully understood. We investigated whether hydrogen sulfide (H2S), an endogenously produced metabolite believed to be an O2 sensor/transducer of vasomotor tone, contributes to the increased Rsys of anoxic red-eared slider turtles ( Trachemys scripta ). Vascular infusion of the H2S donor NaHS in anesthetized turtles at 21°C and fully recovered normoxic turtles at 5°C and 21°C revealed H2S to be a potent vasoconstrictor of the systemic circulation. Likewise, wire myography of isolated turtle mesenteric and pulmonary arteries demonstrated H2S to mediate an anoxia-induced constriction. Intriguingly, however, NaHS did not exert vasoconstrictory effects during anoxia (6 h at 21°C; 14 days at 5°C) when plasma H2S concentration, estimated from the colorimetric measurement of plasma acid-labile sulfide concentration, likely increased by ∼3- and 4-fold during anoxia at 21°C, and 5°C, respectively. Yet, blockade of endogenous H2S production by DL-propargylglycine or hydroxylamine (0.44 mmol/kg) partially reversed the decreased systemic conductance ( Gsys) exhibited by 5°C anoxic turtles. These findings suggest that the signal transduction pathway of H2S-mediated vasoactivity is either maximally activated in the systemic circulation of anoxic turtles and/or that it is oxygen dependent.

Hypoxia increases ROS signaling and cytosolic Ca(2+) in pulmonary artery smooth muscle cells of mouse lungs slices

Precapillary arteries constrict during alveolar hypoxia in a response known as hypoxic pulmonary vasoconstriction (HPV). The mechanism by which pulmonary arterial smooth muscle cells (PASMCs) detect a decrease in Po(2) and trigger contraction is not fully understood. Previous studies in cultured PASMCs show that hypoxia induces an increase in reactive oxygen species (ROS) production, but these results may not reflect responses of PASMCs in their native tissue environment. We therefore assessed hypoxia-induced changes in cytosolic ROS in PASMCs of precision-cut mouse lung slices expressing the redox-sensitive protein, RoGFP. Superfusion of lung slices with hypoxic media (1.5% O(2)) resulted in a significant oxidation of RoGFP from normoxic baseline that was attenuated by overexpression of cytosolic catalase. Hypoxic superfusion also increased [Ca(2+)](i) above normoxic baseline; this response was significantly attenuated by cytosolic catalase overexpression or by the administration of EUK134, a synthetic SOD-catalase mimetic. The hypoxia-induced increase in [Ca(2+)](i) was abolished in the absence of extracellular Ca(2+), indicating that ROS signals trigger entry of extracellular calcium. Collectively, these results indicate that an increase in cytosolic ROS signaling is required for the increase in [Ca(2+)](i) in PASMCs in precision-cut mouse lung slices during the acute HPV response.

Hypoxia sensing in the fetal chicken femoral artery is mediated by the mitochondrial electron transport chain

Vascular hypoxia sensing is transduced into vasoconstriction in the pulmonary circulation, whereas systemic arteries dilate. Mitochondrial electron transport chain (mETC), reactive O(2) species (ROS), and K(+) channels have been implicated in the sensing/signaling mechanisms of hypoxic relaxation in mammalian systemic arteries. We aimed to investigate their putative roles in hypoxia-induced relaxation in fetal chicken (19 days of incubation) femoral arteries mounted in a wire myograph. Acute hypoxia (Po(2) approximately 2.5 kPa) relaxed the contraction induced by norepinephrine (1 microM). Hypoxia-induced relaxation was abolished or significantly reduced by the mETC inhibitors rotenone (complex I), myxothiazol and antimycin A (complex III), and NaN(3) (complex IV). The complex II inhibitor 3-nitroproprionic acid enhanced the hypoxic relaxation. In contrast, the relaxations mediated by acetylcholine, sodium nitroprusside, or forskolin were not affected by the mETC blockers. Hypoxia induced a slight increase in ROS production (as measured by 2,7-dichlorofluorescein-fluorescence), but hypoxia-induced relaxation was not affected by scavenging of superoxide (polyethylene glycol-superoxide dismutase) or H(2)O(2) (polyethylene glycol-catalase) or by NADPH-oxidase inhibition (apocynin). Also, the K(+) channel inhibitors tetraethylammonium (nonselective), diphenyl phosphine oxide-1 (voltage-gated K(+) channel 1.5), glibenclamide (ATP-sensitive K(+) channel), iberiotoxin (large-conductance Ca(2+)-activated K(+) channel), and BaCl(2) (inward-rectifying K(+) channel), as well as ouabain (Na(+)-K(+)-ATPase inhibitor) did not affect hypoxia-induced relaxation. The relaxation was enhanced in the presence of the voltage-gated K(+) channel blocker 4-aminopyridine. In conclusion, our experiments suggest that the mETC plays a critical role in O(2) sensing in fetal chicken femoral arteries. In contrast, hypoxia-induced relaxation appears not to be mediated by ROS or K(+) channels.

Synthesis and electrochemical behavior of triazole-containing nicotinamide adenine dinucleotide analogs

The coupling of 2′,3′-di-O-acetyl nicotinamide mononucleotide with 3-butyn-1-ol in the presence of 2,4,6-triisopropylbenzenesulfonyl chloride quantitatively afforded a terminal alkyne-containing intermediate. Furthermore, copper(I)-mediated Huisgen [3 + 2] cycloaddition with a series of azido compounds in a two-phase solvent system gave eight triazole-containing nicotinamide adenine dinucleotide analogs with yields over 88%. The cyclic voltammetric behaviors of these novel analogs were investigated with a glassy carbon electrode, and structural features of these analogs on their electrochemical properties were briefly discussed.

Hypoxia selectively inhibits KCNA5 channels in pulmonary artery smooth muscle cells

Acute hypoxia induces pulmonary vasoconstriction and chronic hypoxia causes pulmonary vascular remodeling characterized by significant vascular medial hypertrophy. Electromechanical and pharmacomechanical mechanisms are involved in regulating pulmonary vasomotor tone, while changes in cytosolic Ca2+ concentration ([Ca2+](cyt)) are an important signal in regulating contraction and proliferation of pulmonary artery smooth muscle cells (PASMC). Hypoxia-induced increases in [Ca2+](cyt) are, in part, mediated by selective inhibition of voltage-gated K+ (Kv) channels in PASMC. Kv1.5, encoded by the KCNA5 gene, is a Kv channel alpha subunit that forms functional homotetrameric and heterotetrameric Kv channels in PASMC. Activity of Kv channels contributes to the regulation of resting membrane potential. Overexpression of the human KCNA5 gene in rat PASMC and other cell types increases whole-cell Kv currents and causes membrane hyperpolarization. However, acute hypoxia only reduced Kv currents in KCNA5-transfected PASMC. These results provide compelling evidence that Kv1.5 is an important hypoxia-sensitive Kv channel in PASMC, contributing to regulation of membrane potential and intracellular Ca2+ homeostasis during hypoxia. This hypoxia-sensitive mechanism essential for inhibiting Kv1.5 channel activity is exclusively present in PASMC.

Mitochondrial-derived hydrogen peroxide inhibits relaxation of bovine coronary arterial smooth muscle to hypoxia through stimulation of ERK MAP kinase

Mitochondrial reactive oxygen species (ROS) are potentially important in vascular oxygen-sensing mechanisms because hypoxia appears to be a stimulus for mitochondrial ROS generation; however, scavenging of endogenous ROS does not alter relaxation of endothelium-denuded bovine coronary arteries (BCA) to hypoxia. The purpose of this study was to investigate the influence of increasing mitochondrial ROS on the relaxation of BCA to hypoxia. Increasing mitochondrial superoxide with inhibitors of electron transport (10 microM rotenone and antimycin) and by opening mitochondrial ATP-dependent K+ channels with 100 microM diazoxide were observed in this study to attenuate relaxation of BCA precontracted with 30 mM KCl to hypoxia by 68-76% and 38%, respectively. This effect of rotenone is not prevented by inhibiting NADPH oxidase (Nox) activation or scavenging superoxide with Peg-SOD; however, it is reversed 85% and 26% by increasing the consumption of intracellular peroxide by 0.1 mM ebselen and 32.5 U/ml Peg-catalase. Because inhibition of extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase (10 microM PD-98059), but not src kinase or rho kinase, also reverses the effects of rotenone by 69%, the peroxide-elicited force-enhancing effects of ERK appear to be attenuating the response to hypoxia. Rotenone increased the phosphorylation of ERK (by 163%). Activation of ERK in BCA with 0.1 mM peroxide or endogenous peroxide generated by stimulating Nox2 with a stretch treatment or contraction with 100 nM U-46619 also attenuated relaxation to hypoxia. Thus coronary arterial relaxation to hypoxia may be attenuated by pathophysiological conditions associated with increased peroxide generation by mitochondria or other sources that stimulate ERK.

Pathological role of hypoxia in Alzheimer's disease

The majority cases of Alzheimer's disease (AD) are sporadic late-onset form not being linked to APP and PS1 gene mutations. It is believed that the environmental risk factors play an important role in the onset and development of AD. Patients suffering from cerebral ischemia and stroke in which hypoxic conditions occur are much more susceptible to AD. Increasing evidence suggests that hypoxia facilitates the pathogenesis of AD through accelerating the accumulation of Abeta, increasing the hyperphosphorylation of tau, impairing the normal functions of blood-brain barrier, and promoting the degeneration of neurons. Further investigations into the relationship between hypoxia and AD may open the avenue for effective preservation and pharmacological treatments of this neurodegenerative disease.

NADPH oxidases and reactive oxygen species at different stages of chronic hypoxia-induced pulmonary hypertension in newborn piglets

Recently, we reported that reactive oxygen species (ROS) generated by NADPH oxidase (NOX) contribute to aberrant responses in pulmonary resistance arteries (PRAs) of piglets exposed to 3 days of hypoxia (Am J Physiol Lung Cell Mol Physiol 295: L881-L888, 2008). An objective of the present study was to determine whether NOX-derived ROS also contribute to altered PRA responses at a more advanced stage of pulmonary hypertension, after 10 days of hypoxia. We further wished to advance knowledge about the specific NOX and antioxidant enzymes that are altered at early and later stages of pulmonary hypertension. Piglets were raised in room air (control) or hypoxia for 3 or 10 days. Using a cannulated artery technique, we found that treatments with agents that inhibit NOX (apocynin) or remove ROS [an SOD mimetic (M40403) + polyethylene glycol-catalase] diminished responses to ACh in PRAs from piglets exposed to 10 days of hypoxia. Western blot analysis showed an increase in expression of NOX1 and the membrane fraction of p67phox. Expression of NOX4, SOD2, and catalase were unchanged, whereas expression of SOD1 was reduced, in arteries from piglets raised in hypoxia for 3 or 10 days. Markers of oxidant stress, F(2)-isoprostanes, measured by gas chromatography-mass spectrometry, were increased in PRAs from piglets raised in hypoxia for 3 days, but not 10 days. We conclude that ROS derived from some, but not all, NOX family members, as well as alterations in the antioxidant enzyme SOD1, contribute to aberrant PRA responses at an early and a more progressive stage of chronic hypoxia-induced pulmonary hypertension in newborn piglets.

Effect of moderate hypoxia/reoxygenation on mitochondrial adaptation to acute severe hypoxia

In an experimental model, it was shown that repetitive periods of hypoxia/reoxygenation (H/R) [5 cycles of 5 min hypoxia (12% O(2) in N(2) ) followed by 15 min normoxia, daily for three weeks] attenuated basal and stimulated in vitro lipid peroxidation, as well as H(2)O(2) production in liver and brain mitochondria of rats exposed to acute severe hypoxia. Adaptation to moderate H/R enhanced in mitochondria the production and activity of reactive oxygen species scavengers, such as glutathione, manganese superoxide dismutase, glutathione peroxidase, and glutathione-S-transferase. It was demonstrated that the maintenance of GSH-redox cycle by activation of glutathione reductase and NADP(+) -dependent isocitrate dehydrogenase is an integral part of the biochemical adaptive mechanism of oxidative tolerance to new damaging factor. Brain mitochondria showed more sensitivity to oxidative stress than liver mitochondria, and long-lasting sessions of H/R affect differentially their pro-/antioxidant homeostasis.

Oral antioxidant supplementation does not prevent acute mountain sickness: double blind, randomized placebo-controlled trial

BACKGROUND

Acute mountain sickness may be caused by cerebrovascular fluid leakage due to oxidative damage to the endothelium. This may be reduced by oral antioxidant supplementation.

AIM

To assess the effectiveness of antioxidant supplementation for the prevention of acute mountain sickness (AMS).

DESIGN

A parallel-group double blind, randomized placebo-controlled trial.

METHODS

The study was conducted in a university clinical research facility and a high altitude research laboratory. Eighty-three healthy lowland volunteers ascended to 5200 m on the Apex 2 high altitude research expedition. The treatment group received a daily dose of 1 g l-ascorbic acid, 400 IU of alpha-tocopherol acetate and 600 mg of alpha-lipoic acid (Cultech Ltd., Wales, UK) in four divided doses. Prevalence of AMS was measured using the Lake Louise Consensus score sheet (LLS). Secondary outcomes were AMS severity measured using a novel visual analogue scale, arterial oxygen saturation and pulmonary artery systolic pressure (PASP).

RESULTS

Forty-one subjects were allocated to the antioxidant group, and 42 to the placebo group. There was no difference in AMS incidence or severity between the antioxidant and placebo groups using the LLS at any time at high altitude. At the pre-determined comparison point at Day 2 at 5200 m, 69% of the antioxidant group (25/36) and 66% of the placebo group (23/35) had AMS using the LLS criteria (P = 0.74). No differences were observed between the groups for PASP, oxygen saturation, presence of a pericardial effusion or AMS assessed by VAS.

CONCLUSION

This trial found no evidence of benefit from antioxidant supplementation at high altitude.

TRIAL REGISTRATION NUMBER

NCT00664001.

Mechanisms of hypoxic pulmonary vasoconstriction and their roles in pulmonary hypertension: new findings for an old problem

Hypoxic pulmonary vasoconstriction (HPV) normally optimises ventilation-perfusion matching in the lung, but leads to pulmonary hypertension (PH) under conditions of global hypoxia. The past few years have provided some major advances in our understanding of this complex phenomenon, but significant controversy remains concerning many of the key underlying mechanisms. On balance, recent evidence is most consistent with an elevation in mitochondria-derived reactive oxygen species as a key event for initiation of HPV, with consequent Ca2+ release from intracellular ryanodine-sensitive stores, although the activation pathways and molecular identity of the associated Ca2+ entry pathways remain unclear. Recent studies have also raised our perception of the critical role played by Rho kinase (ROCK) in both sustained HPV and the development of PH, further promoting ROCK and the pathways regulating its activity and expression as important therapeutic targets.

Transcriptional and post-transcriptional regulation of gene expression in submerged root cells of maize

Maize survival under the anaerobic stress due to submergence conditions is dependent on complex metabolic, physiological and morphological adaptation strategies. Here, we focus on gene expression regulation at the transcriptional and post-transcriptional level in submerged maize root cells. Early in progressive oxygen deprivation, root cells sense the low oxygen signal to trigger expressions of TF genes, anaerobic response genes and miRNA genes. The induced TFs, in turn, promote a broad spectrum of responses from morphogenetic to metabolic; these responses occur at later stages of the stress treatment. The selective translation of anaerobically induced transcripts and selective degradation of some APs are also suggested to be an important regulatory mechanism. In addition, miRNAs are possibly transcriptionally regulated in submerged root cells and involved in post-transcriptional control of target genes. Thus, regulation of gene expression in response to low oxygen involves in significant transcriptional and post-transcriptional control.

Peroxide generation by p47phox-Src activation of Nox2 has a key role in protein kinase C-induced arterial smooth muscle contraction

Protein kinase C (PKC) stimulation of NAD(P)H oxidases (Nox) is an important component of multiple vascular disease processes; however, the relationship between oxidase activation and the regulation of vascular smooth muscle contraction by PKC remains poorly understood. Therefore, we examined the signaling cascade of PKC-elicited Nox activation and the role of superoxide and hydrogen peroxide in mediating PKC-induced vascular contraction. Endothelium-denuded bovine coronary arteries showed a PKC-dependent basal production of lucigenin (5 muM)-detected Nox oxidase-derived superoxide, which was stimulated fourfold by PKC activation with 10 muM phorbol 12,13-dibutyrate (PDBu). PDBu appeared to increase superoxide generation by Nox2 through both p47(phox) and peroxide-dependent Src activation mechanisms based on the actions of inhibitors, properties of Src phosphorylation, and the loss of responses in aorta from mice deficient in Nox2 and p47(phox). The actions of inhibitors of contractile regulating mechanisms, scavengers of superoxide and peroxide, and responses in knockout mouse aortas suggest that a major component of the contraction elicited by PDBu appeared to be mediated through peroxide derived from Nox2 activation stimulating force generation through Rho kinase and calmodulin kinase-II mechanisms. Superoxide generated by PDBu also attenuated relaxation to nitroglycerin. Peroxide-derived from Nox2 activation by PKC appeared to be a major contributor to the thromboxane A(2) receptor agonist U46619 (100 nM)-elicited contraction of coronary arteries. Thus a p47(phox) and Src kinase activation of peroxide production by Nox2 appears to be an important contributor to vascular contractile mechanisms mediated through activation of PKC.

Reactive oxygen species and the control of vascular function

This article summarizes perspectives on how reactive oxygen species (ROS) and redox signaling mechanisms participate in regulating vascular smooth muscle function that have resulted from our studies over the past 25 years in areas including oxygen sensing and the regulation of cGMP production by soluble guanylate cyclase (sGC) that were presented in the Robert M. Berne Distinguished Lectureship at the 2008 Experimental Biology Meeting. It considers mechanisms controlling the activity of sources of ROS including Nox oxidases and mitochondria by physiological stimuli, vascular diseases processes, and metabolic mechanisms linked to NAD(P)H redox and hypoxia. Metabolic interactions of individual ROS such as hydrogen peroxide with cellular peroxide metabolizing enzymes are viewed as some of the most sensitive ways of influencing cellular signaling systems. The control of cytosolic NADPH redox also seems to be a major contributor to bovine coronary arterial relaxation to hypoxia, where its oxidation functions to coordinate the lowering of intracellular calcium, whereas increased cytosolic NADPH generation in pulmonary arteries appears to maintain elevated Nox oxidase activity, and relaxation to hydrogen peroxide, which is attenuated by hypoxia. The sensitivity of sGC to nitric oxide seems to be regulated by thiol and heme redox systems controlled by cytosolic NADPH. Heme biosynthesis and metabolism are also important factors regulating the sGC system. The signaling pathways controlling oxidases and their colocalization with redox-regulated systems enables selective activation of numerous regulatory mechanisms influencing vascular function in physiological processes and the progression of aging-associated vascular diseases.

Pulmonary artery hypertension in a child with MELAS due to a point mutation of the mitochondrial tRNA((Leu)) gene (m.3243A>G)

Although linked with cardiac dysfunction, the association of MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) and pulmonary artery hypertension (PAH) has not been previously described. PAH and right ventricular heart failure were identified by echocardiography in a 3-year-old boy with a history of hypotonia, microcephaly and developmental delay. He initially presented with a 10-day history of dyspnoea, dependent oedema and reduced oral intake. Lactic acidosis was noted on serial arterial blood sampling and cerebrospinal fluid. Muscle biopsy demonstrated cytochrome-c oxidase-positive 'ragged-red' fibres consistent with MELAS; subsequent analyses revealed the m.3243A>G point mutation most commonly associated with MELAS. The mutation was heteroplasmic, representing 92% of the total mtDNA from a lung sample. Nitric oxide and epoprostenol were administered without significant clinical or echocardiographic improvement of his PAH. A 'mitochondrial cocktail' including biotin, riboflavin, carnitine and coenzyme Q10 also was provided. Five months after presentation, he developed seizures; MRI imaging of his brain demonstrated multiple focal lesions. His clinical status worsened with increasing cardiopulmonary failure. He died two months later. Although therapy for both MELAS and PAH remains limited, recent investigations suggest a beneficial role for l-arginine in both conditions, implying a possible common pathophysiology. Mitochondrial diseases such as MELAS should be considered in cases of idiopathic PAH, particularly when associated with multisystem involvement including short stature, hearing loss, renal dysfunction, retinopathy, diabetes mellitus, migraines, seizures, ophthalmoplegia, fatigability and weakness.

Constriction of pulmonary artery by peroxide: role of Ca2+ release and PKC

Reactive oxygen species are implicated in pulmonary hypertension and hypoxic pulmonary vasoconstriction. We examined the effects of low concentrations of peroxide on intrapulmonary arteries (IPA). IPAs from Wistar rats were mounted on a myograph for recording tension and estimating intracellular Ca2+ using Fura-PE3. Ca2+ sensitization was examined in alpha-toxin-permeabilized IPAs, and phosphorylation of MYPT-1 and MLC(20) was assayed by Western blot. Peroxide (30 microM) induced a vasoconstriction with transient and sustained components and equivalent elevations of intracellular Ca2+. The transient constriction was strongly suppressed by indomethacin, the TP-receptor antagonist SQ-29584, and the Rho kinase inhibitor Y-27632, whereas sustained constriction was unaffected. Neither vasoconstriction nor elevation of intracellular Ca2+ was affected by removal of extracellular Ca2+, whereas dantrolene suppressed the former and ryanodine abolished the latter. Peroxide-induced constriction of permeabilized IPAs was unaffected by Y-27632 but abolished by PKC inhibitors; these also suppressed constriction in intact IPAs. Peroxide caused translocation of PKCalpha, but had no significant effect on MYPT-1 or MLC(20) phosphorylation. We conclude that in IPAs peroxide causes transient release of vasoconstrictor prostanoids, but sustained constriction is associated with release of Ca2+ from ryanodine-sensitive stores and a PKC-dependent but Rho kinase- and MLC(20)-independent constrictor mechanism.

Effects of mitochondrial poisons on glutathione redox potential and carotid body chemoreceptor activity

Low oxygen sensing in chemoreceptor cells involves the inhibition of specific plasma membrane K(+) channels, suggesting that mitochondria-derived reactive oxygen species (ROS) link hypoxia to K(+) channel inhibition, subsequent cell depolarization and activation of neurotransmitter release. We have used several mitochondrial poisons, alone and in combination with the antioxidant N-acetylcysteine (NAC), and quantify their capacity to alter GSH/GSSG levels and glutathione redox potential (E(GSH)) in rat diaphragm. Selected concentrations of mitochondrial poisons with or without NAC were tested for their capacity to activate neurotransmitter release in chemoreceptor cells and to alter ATP levels in intact rat carotid body (CB). We found that rotenone (1 microM), antimycin A (0.2 microg/ml) and sodium azide (5mM) decreased E(GSH); NAC restored E(GSH) to control values. At those concentrations mitochondrial poisons activated neurotransmitter release from CB chemoreceptor cells and decreased CB ATP levels, NAC being ineffective to modify these responses. Additional experiments with 3-nitroprionate (5mM), lower concentrations of rotenone and dinitrophenol revealed variable relationships between E(GSH) and chemoreceptor cell neurotransmitter release responses and ATP levels. These findings indicate a lack of correlation between mitochondrial-generated modifications of E(GSH) and chemoreceptor cells activity. This lack of correlation renders unlikely that alteration of mitochondrial production of ROS is the physiological pathway chemoreceptor cells use to signal hypoxia.

Effects of intermittent hypoxia different regimes on mitochondrial lipid peroxidation and glutathione-redox balance in stressed rats

The purpose of this study was to compare the influence of two regimes of intermittent hypoxia (IH) [repetitive 5 cycles of 5 min hypoxia (7% O2 or 12% O2 in N2) followed by 15 min normoxia, daily for three weeks] on oxidative stress protective systems in liver mitochondria. To estimate the effectiveness of hypoxia adaptation at the early and late preconditioning period, we exposed rats to acute 6-h immobilization at the 1st and 45th days after cessation of IH. We showed that severity of hypoxic episodes during IH might initiate different adaptive programs. Moderate hypoxia during IH prevents mitochondrial glutathione pool depletion induced by immobilization stress, maintains GSH-redox cycle via activation of glutathione peroxidase, glutathione-S-transferase, glutathione reductase, NADP+-dependent isocitrate dehydrogenase, and increases Mn-SOD activity. Such regimen of hypoxic preconditioning caused the decrease of mitochondrial superoxide anion generation as well as of basal and stimulated in vitro lipid peroxidation and this protective effect remained for 45 days under renormoxic conditions. Hypoxic adaptation in a more severe regimen exerted beneficial effects on the mitochondrial antioxidant defense system only at its later phase.

Effects of hypoxia on relationships between cytosolic and mitochondrial NAD(P)H redox and superoxide generation in coronary arterial smooth muscle

Since controversy exists on how hypoxia influences vascular reactive oxygen species (ROS) generation, and our previous work provided evidence that it relaxes endothelium-denuded bovine coronary arteries (BCA) in a ROS-independent manner by promoting cytosolic NADPH oxidation, we examined how hypoxia alters relationships between cytosolic and mitochondrial NAD(P)H redox and superoxide generation in BCA. Methods were developed to image and interpret the effects of hypoxia on NAD(P)H redox based on its autofluorescence in the cytosolic, mitochondrial, and nuclear regions of smooth muscle cells isolated from BCA. Aspects of anaerobic glycolysis and cytosolic NADH redox in BCA were assessed from measurements of lactate and pyruvate. Imaging changes in mitosox and dehydroethidium fluorescence were used to detect changes in mitochondrial and cytosolic-nuclear superoxide, respectively. Hypoxia appeared to increase mitochondrial and decrease cytosolic-nuclear superoxide under conditions associated with increased cytosolic NADH (lactate/pyruvate), mitochondrial NAD(P)H, and hyperpolarization of mitochondria detected by tetramethylrhodamine methyl-ester perchlorate fluorescence. Rotenone appeared to increase mitochondrial NAD(P)H and superoxide, suggesting hypoxia could increase superoxide generation by complex I. However, hypoxia decreased mitochondrial superoxide in the presence of contraction to 30 mM KCl, associated with decreased mitochondrial NAD(P)H. Thus, while hypoxia augments NAD(P)H redox associated with increased mitochondrial superoxide, contraction with KCl reverses these effects of hypoxia on mitochondrial superoxide, suggesting mitochondrial ROS increases do not mediate hypoxic relaxation in BCA. Since hypoxia lowers pyruvate, and pyruvate inhibits hypoxia-elicited relaxation and NADPH oxidation in BCA, mitochondrial control of pyruvate metabolism associated with cytosolic NADPH redox regulation could contribute to sensing hypoxia.

Microcirculation and oxidative stress

The microcirculation is a complex and integrated system, transporting oxygen and nutrients to the cells. The key component of this system is the endothelium, contributing to the local balance between pro and anti-inflammatory mediators, hemostatic balance, as well as vascular permeability and cell proliferation. A constant shear stress maintains vascular endothelium homeostasis while perturbed shear stress leads to changes in secretion of vasodilator and vasoconstrictor agents. Increased oxidative stress is a major pathogenetic mechanism of endothelial dysfunction by decreasing NO bioavailability, promoting inflammation and participating in activation of intracellular signals cascade, so influencing ion channels activation, signal transduction pathways, cytoskeleton remodelling, intercellular communication and ultimately gene expression. Targeting the microvascular inflammation and oxidative stress is a fascinating approach for novel therapies in order to decrease morbidity and mortality of chronic and acute diseases.

Mitochondria-dependent regulation of Kv currents in rat pulmonary artery smooth muscle cells

Voltage-gated K⁺ (Kv) channels are important in the regulation of pulmonary vascular function having both physiological and pathophysiological implications. The pulmonary vasculature is essential for reoxygenation of the blood, supplying oxygen for cellular respiration. Mitochondria have been proposed as the major oxygen-sensing organelles in the pulmonary vasculature. Using electrophysiological techniques and immunofluorescence, an interaction of the mitochondria with Kv channels was investigated. Inhibitors, blocking the mitochondrial electron transport chain at different complexes, were shown to have a dual effect on Kv currents in freshly isolated rat pulmonary arterial smooth muscle cells (PASMCs). These dual effects comprised an enhancement of Kv current in a negative potential range (manifested as a 5- to 14-mV shift in the Kv activation to more negative membrane voltages) with a decrease in current amplitude at positive potentials. Such effects were most prominent as a result of inhibition of Complex III by antimycin A. Investigation of the mechanism of antimycin A-mediated effects on Kv channel currents (I_Kv) revealed the presence of a mitochondria-mediated Mg²⁺ and ATP-dependent regulation of Kv channels in PASMCs, which exists in addition to that currently proposed to be caused by changes in intracellular reactive oxygen species.

Mitochondrial pathophysiology, reactive oxygen species, and cardiovascular diseases

This article discusses mitochondrial pathophysiology, reactive oxygen species, and cardiovascular diseases. Mitochondrial respiratory chains are responsible for energy metabolism/ATP production through the tricyclic antidepressant cycle, coupling of oxidative phosphorylation, and electron transfer. The mitochondrion produces reactive oxygen species as "side products" of respiration. The mitochondrial derived reactive oxygen species is involved in the pathogenesis of various clinical disorders including heart failure, hypoxia, ischemia/reperfusion injury, diabetes, neurodegenerative diseases, and the physiologic process of aging. Observational and mechanistical studies of these pathologic roles of mitochondria are discussed in depth in this article.

Generation of oxidants by hypoxic human pulmonary and coronary smooth-muscle cells

BACKGROUND

Pulmonary vasoconstriction in response to hypoxia is unusual inasmuch as local exposure of nonpulmonary vasculature to hypoxia results in vasodilation. It has been suggested that pulmonary artery smooth-muscle cells may relax in response to intracellular generation of reactive oxygen species (ROS) and that the production of ROS decreases under hypoxia. However, other workers report increased ROS production in human pulmonary artery smooth-muscle cells (HPASMC) during hypoxia.

METHODS

Using dihydrodichlorofluorescein diacetate, dihydroethidium, and Amplex Red (Molecular Probes; Eugene, OR), we estimated ROS generation by confluent primary cultures of HPASMC and human coronary artery smooth-muscle cells (HCASMC) under normoxia (20%) and acute hypoxia (5%).

RESULTS

All three assay systems showed that HPASMC production of ROS is decreased under hypoxia and to a greater extent than the decrease in ROS production by HCASMC. A substantially greater percentage of normoxic ROS production by HPASMC is mitochondrial (> 60%) compared to HCASMC (< 30%).

CONCLUSIONS

These results support the conclusion that ROS generation decreases, rather than increases, in HPASMC during hypoxia. However, as ROS production also decreases in HCASMC during hypoxia, the reason for the opposite change in vascular tone is not yet apparent.

NAD+/NADH and NADP+/NADPH in cellular functions and cell death: regulation and biological consequences

Accumulating evidence has suggested that NAD (including NAD+ and NADH) and NADP (including NADP+ and NADPH) could belong to the fundamental common mediators of various biological processes, including energy metabolism, mitochondrial functions, calcium homeostasis, antioxidation/generation of oxidative stress, gene expression, immunological functions, aging, and cell death: First, it is established that NAD mediates energy metabolism and mitochondrial functions; second, NADPH is a key component in cellular antioxidation systems; and NADH-dependent reactive oxygen species (ROS) generation from mitochondria and NADPH oxidase-dependent ROS generation are two critical mechanisms of ROS generation; third, cyclic ADP-ribose and several other molecules that are generated from NAD and NADP could mediate calcium homeostasis; fourth, NAD and NADP modulate multiple key factors in cell death, such as mitochondrial permeability transition, energy state, poly(ADP-ribose) polymerase-1, and apoptosis-inducing factor; and fifth, NAD and NADP profoundly affect aging-influencing factors such as oxidative stress and mitochondrial activities, and NAD-dependent sirtuins also mediate the aging process. Moreover, many recent studies have suggested novel paradigms of NAD and NADP metabolism. Future investigation into the metabolism and biological functions of NAD and NADP may expose fundamental properties of life, and suggest new strategies for treating diseases and slowing the aging process.

Intravital microscopy of the murine pulmonary microcirculation

Intravital microscopy (IVM) is considered as the gold standard for in vivo investigations of dynamic microvascular regulation. The availability of transgenic and knockout animals has propelled the development of murine IVM models for various organs, but technical approaches to the pulmonary microcirculation are still scarce. In anesthetized and ventilated BALB/c mice, we established a microscopic access to the surface of the right upper lung lobe by surgical excision of a window of 7- to 10-mm diameter from the right thoracic wall. The window was covered by a transparent polyvinylidene membrane and sealed with α-cyanoacrylate. Removal of intrathoracic air via a transdiaphragmal intrapleural catheter coupled the lung surface to the window membrane. IVM preparations were hemodynamically stable for at least 120 min, with mean arterial blood pressure above 70 mmHg, and mean arterial Po2 and arterial Pco2 in the range of 90–100 Torr and 30–40 Torr, respectively. Imaged lungs did not show any signs of acute lung injury or edema. Following infusion of FITC dextran, subpleural pulmonary arterioles and venules of up to 50-μm diameter and alveolar capillary networks could be visualized during successive expiratory plateau phases over a period of at least 2 h. Vasoconstrictive responses to hypoxia (11% O2) or infusion of the thromboxane analog U-46619 were prominent in medium-sized arterioles (30- to 50-μm diameter), minor in small arterioles <30 μm, and absent in venules. The presented IVM model may constitute a powerful new tool for investigations of pulmonary microvascular responses in mice.

Chapter 13. Oxygen as a direct and indirect biological determinant in the vasculature

A fundamental function of the vasculature is to deliver oxygen to tissues and organs. The cells that make up the vasculature also require oxygen, and are acted upon by oxygen in direct and indirect ways that can have significant effects on acute and chronic vascular function and morphology. The role that oxygen, or its absence, plays in defining the biology of the vasculature is thus of critical importance, yet remains an area about which there are many gaps in knowledge and understanding. Oxygen-associated paracrine mechanisms can drive vascular processes such as angiogenesis. The vasculature can also directly sense blood oxygen levels and differentially translate this information into rapid vasoconstriction responses in some vascular beds, and vasodilation in others. Furthering our understanding of how oxygen and hypoxia affect the vasculature may lead to greater insights into the mechanisms and pathogenesis of disease processes involving the vasculature, and lead to new therapeutic paradigms.

Direct eicosanoid profiling of the hypoxic lung by comprehensive analysis via capillary liquid chromatography with dual online photodiode-array and tandem mass-spectrometric detection

Eicosanoids are arachidonic acid-derived mediators, with partly contradictory, incompletely elucidated actions. Thus, epoxyeicosatrienoic acids (EETs) are controversially discussed as putative vasodilatative endothelium-derived hyperpolarizing factors in the cardiovascular compartment but reported as vasoconstrictors in the lung. Inconsistent findings concerning eicosanoid physiology may be because previous methods were lacking sensitivity, identification reliability, and/or have focused on special eicosanoid groups only, ignoring the overall mediator context, and thus limiting the correlation accuracy between autacoid formation and bioactivity profile. Therefore, we developed an approach which enables the simultaneous assessment of 44 eicosanoids, including all representatives of the arachidonic acid cascade, i.e., cytochrome P450, lipoxygenase, cyclooxygenase products, and free isoprostanes as in vivo markers of oxidative stress, in one 50-minute chromatographic run. The approach combines (i) source-specific sample extraction, (ii) rugged isocratic and high-sensitivity capillary liquid-chromatographic separation, and (iii) reliable dual online photodiode-array and electrospray ionization tandem mass-spectrometric identification and quantitation. High sensitivity with limits of quantification in the femtogram range was achieved by use of capillary columns with typical high peak efficiency, due to small inner diameters, and virtually complete substance transfer to the mass spectrometer, due to flow rates in the low microliter range, instead of large inner diameter columns with low chromatographic signal and only partial analyte transfer employed by previous methods. This expeditious, global and sensitive technique provides the prerequisite for new, accurate insights regarding the physiology of specific mediators, for example EETs, in the context of all relevant vasoactive autacoids under varying conditions of oxidative stress by direct comparison of all eicosanoid generation profiles. Indeed, application of comprehensive "eicoprofiling" to hypoxically ventilated rabbit lungs revealed at a glance the enhanced biosynthesis of free EETs in the overall mediator generation context, thus suggesting their hypothetical contribution to hypoxic pulmonary vasoconstriction.

Activation of Toll-like receptor 2 impairs hypoxic pulmonary vasoconstriction in mice

Toll-like receptors (TLRs) mediate inflammation in sepsis, but their role in sepsis-induced respiratory failure is unknown. Hypoxic pulmonary vasoconstriction (HPV) is a unique vasoconstrictor response that diverts blood flow away from poorly ventilated lung regions. HPV is impaired in sepsis and after challenge with the TLR4 agonist lipopolysaccharide (LPS). Unlike TLR4 agonists, which are present only in Gram-negative bacteria, TLR2 agonists are ubiquitously expressed in all of the major classes of microorganisms that cause sepsis, including both Gram-positive and Gram-negative bacteria and fungi. We tested the hypothesis that (S)-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-N-palmitoyl-(R)-Cys-(S)-Ser(S)-Lys(4)-OH, trihydrochloride (Pam3Cys), a TLR2 agonist, impairs HPV and compared selected pulmonary and systemic effects of Pam3Cys vs. LPS. HPV was assessed 22 h after challenge with saline, Pam3Cys, or LPS by measuring the increase in the pulmonary vascular resistance of the left lung before and during left lung alveolar hypoxia produced by left mainstem bronchus occlusion (LMBO). Additional endpoints included arterial blood gases during LMBO, hemodynamic parameters, weight loss, temperature, physical appearance, and several markers of lung inflammation. Compared with saline, challenge with Pam3Cys caused profound impairment of HPV, reduced systemic arterial oxygenation during LMBO, weight loss, leukopenia, and lung inflammation. In addition to these effects, LPS-challenged mice had lower rectal temperatures, metabolic acidosis, and were more ill appearing than Pam3Cys-challenged mice. These data indicate that TLR2 activation impairs HPV and induces deleterious systemic effects in mice and suggest that TLR2 pathways may be important in sepsis-induced respiratory failure.

Oxygen sensors in context

The ability to adapt to changes in the availability of O2 provides a critical advantage to all O2-dependent lifeforms. In mammals it allows optimal matching of the O2 requirements of the cells to ventilation and O2 delivery, underpins vital changes to the circulation during the transition from fetal to independent, air-breathing life, and provides a means by which dysfunction can be limited or prevented in disease. Certain tissues such as the carotid body, pulmonary circulation, neuroepithelial bodies and fetal adrenomedullary chromaffin cells are specialised for O2 sensing, though most others show for example alterations in transcription of specific genes during hypoxia. A number of mechanisms are known to respond to variations in PO2 over the physiological range, and have been proposed to fulfil the function as O2 sensors; these include modulation of mitochondrial oxidative phosphorylation and a number of O2-dependent synthetic and degradation pathways. There is however much debate as to their relative importance within and between specific tissues, whether their O2 sensitivity is actually appropriate to account for their proposed actions, and in particular their modus operandi. This review discusses our current understanding of how these mechanisms may operate, and attempts to put them into the context of the actual PO2 to which they are likely to be exposed. An important point raised is that the overall O2 sensitivity (P50) of any O2-dependent mechanism does not necessarily correlate with that of its O2 sensor, as the coupling function between the two may be complex and non-linear. In addition, although the bulk of the evidence suggests that mitochondria act as the key O2 sensor in carotid body, pulmonary artery and chromaffin cells, the signalling mechanisms by which alterations in their function are translated into a response appear to differ fundamentally, making a global unified theory of O2 sensing unlikely.

Regulation of cGMP-dependent protein kinase-mediated vasodilation by hypoxia-induced reactive species in ovine fetal pulmonary veins

We previously reported that hypoxia attenuates cGMP-dependent protein kinase (PKG)-mediated relaxation in pulmonary vessels (Am J Physiol Lung Cell Mol Physiol 279: L611-L618, 2003). To determine whether hypoxia-induced reactive oxygen and nitrogen species (ROS and RNS, respectively) may be involved in the downregulation of PKG-mediated relaxation, ovine fetal intrapulmonary veins were exposed to 4 h of normoxia or hypoxia, with or without scavengers of ROS [N-acetylcysteine (NAC)] or peroxynitrite (quercetin and Trolox) and preconstricted with endothelin-1. Hypoxia decreased the relaxation response to 8-bromo-cGMP, PKG protein expression, and kinase activity and increased tyrosine nitration in PKG. However, ROS and RNS scavengers prevented these changes. To determine whether increased PKG nitration diminishes PKG activity, pulmonary vein smooth muscle cells (PVSMC) were exposed to shorter-term (30 min) hypoxia, which increased PKG nitration and decreased PKG activity but did not alter PKG protein expression. Increased dihydro-2,7-dichlorofluorescein diacetate (DCFH(2)-DA) fluorescence in PVSMC after 4 h or 30 min of hypoxia was not observed in the presence of NAC, quercetin, or Trolox, suggesting increased ROS and RNS production. Increased PKG nitration and the associated decrease in PKG activity in PVSMC after 30 min of hypoxia were also reversed on reoxygenation. The consequences of PKG nitration were assessed by exposure of purified PKG-Ialpha to peroxynitrite, which caused increased 3-nitrotyrosine immunoreactivity and inhibition of kinase activity. Our data suggest that, after 30 min of hypoxia, reversible covalent modification of PKG by hypoxia-induced reactive species may be an important mechanism by which the relaxation response to cGMP is regulated. However, after 4 h of hypoxia, PKG nitration and decreased PKG expression are involved.

Therapeutic potential of the epidermal growth factor receptor transactivation in hypertension: a convergent signaling pathway of vascular tone, oxidative stress, and hypertrophic growth downstream of vasoactive G-protein-coupled receptors?

The concurrence of enhanced vascular tone, oxidative stress, and hypertrophic growth is a hallmark of hypertension, the condition characterized by sustained elevated blood pressure. However, it is unclear how and why such apparently distinct processes coincide in hypertension. Elevated levels of certain vasoactive G-protein-coupled receptor agonists (such as catecholamines, endothelin-1, and angiotensin II) can explain, at least in part, the development and progression of many hypertensive disorders. Here, we review findings made by other investigators and ourselves suggesting that enhanced vascular tone, oxidative stress, and hypertrophic growth characteristically induced by these agonists involve the transactivation of growth factor receptors. The first step in this transactivation mechanism is agonist-induced activation of metalloproteinase-dependent shedding of growth factors. Shed growth factors then trigger intracellular signaling cascades necessary for growth, production of reactive oxygen species, and maintenance of vascular tone. If this hypothesis is proven generally correct, then transactivation blockers have general therapeutic potential in hypertension regardless of the causative agonist.

Role of the small GTPase RhoA in the hypoxia-induced decrease of plasma membrane Na,K-ATPase in A549 cells

Hypoxia impairs alveolar fluid reabsorption by promoting Na,K-ATPase endocytosis, from the plasma membrane of alveolar epithelial cells. The present study was designed to determine whether hypoxia induces Na,K-ATPase endocytosis via reactive oxygen species (ROS)-mediated RhoA activation. In A549 cells, RhoA activation occurred within 15 minutes of cells exposure to hypoxia. This activation was inhibited in cells infected with adenovirus coding for gluthatione peroxidase (an H2O2 scavenger), in mitochondria depleted (rho0) cells or cells expressing decreased levels of the Rieske iron-sulfur protein (inhibitor of mitochondrial complex III), which suggests a role for mitochondrial ROS. Moreover, exogenous H2O2 treatment during normoxia mimicked the effects of hypoxia on RhoA, further supporting a role for ROS. Cells expressing dominant negative RhoA failed to endocytose the Na,K-ATPase during hypoxia or after H2O2 treatment. Na,K-ATPase endocytosis was also prevented in cells treated with Y-27632, a Rho-associated kinase (ROCK) inhibitor, and in cells expressing dominant negative ROCK. In summary, we provide evidence that in human alveolar epithelial cells exposed to hypoxia, RhoA/ROCK activation is necessary for Na,K-ATPase endocytosis via a mechanism that requires mitochondrial ROS.

Cytosolic NAD(P)H regulation of redox signaling and vascular oxygen sensing

This article considers how regulation of signaling controlled by cytosolic NADPH and NADH redox systems contained within the vascular smooth muscle cell may contribute to coordinating alterations in force generation elicited by acute changes in oxygen tension. Additional important issues considered include defining when oxidases generating reactive oxygen species (ROS), such as Nox oxidases, or ROS metabolizing activities which utilize cytosolic NADH and/or NADPH are key participants in eliciting responses that are observed, and assessing how mitochondria can potentially contribute to the regulation that is seen. Many important signaling mechanisms potentially involved in vascular oxygen sensing such as potassium channels, systems regulating intracellular calcium, and the sensitivity of the contractile apparatus to calcium, and the control of cGMP-mediated relaxation by soluble guanylate cyclase appear to be regulated by cytosolic NAD(P)H redox and or ROS. Differences in the processes controlling the maintenance of cytosolic NADPH redox by the pentose phosphate pathway of glucose metabolism are hypothesized to be a key factor in controlling the expression of a relaxation to hypoxia seen in systemic arteries compared to the hypoxic contractile response observed in pulmonary arterial smooth muscle.

Hydrogen peroxide-induced Ca2+ mobilization in pulmonary arterial smooth muscle cells

Reactive oxygen species (ROS) generated from NADPH oxidases and mitochondria have been implicated as key messengers for pulmonary vasoconstriction and vascular remodeling induced by agonists and hypoxia. Since Ca2+ mobilization is essential for vasoconstriction and cell proliferation, we sought to characterize the Ca2+ response and to delineate the Ca2+ pathways activated by hydrogen peroxide (H2O2) in rat intralobar pulmonary arterial smooth muscle cells (PASMCs). Exogenous application of 10 μM to 1 mM H2O2 elicited concentration-dependent increase in intracellular Ca2+ concentration in PASMCs, with an initial rise followed by a plateau or slow secondary increase. The initial phase was related to intracellular release. It was attenuated by the inositol trisphosphate (IP3) receptor antagonist 2-aminoethyl diphenylborate, ryanodine, or thapsigargin, but was unaffected by the removal of Ca2+ in external solution. The secondary phase was dependent on extracellular Ca2+ influx. It was unaffected by the voltage-gated Ca2+ channel blocker nifedipine or the nonselective cation channel blockers SKF-96365 and La3+, but inhibited concentration dependently by millimolar Ni2+, and potentiated by the Na+/Ca2+ exchange inhibitor KB-R 7943. H2O2 did not alter the rate of Mn2+ quenching of fura 2, suggesting store- and receptor-operated Ca2+ channels were not involved. By contrast, H2O2 elicited a sustained inward current carried by Na+ at −70 mV, and the current was inhibited by Ni2+. These results suggest that H2O2 mobilizes intracellular Ca2+ through multiple pathways, including the IP3- and ryanodine receptor-gated Ca2+ stores, and Ni2+-sensitive cation channels. Activation of these Ca2+ pathways may play important roles in ROS signaling in PASMCs.

Endothelial mitochondria: contributing to vascular function and disease

Disturbances in vascular function contribute to the development of several diseases of increasing prevalence and thereby contribute significantly to human mortality and morbidity. Atherosclerosis, diabetes, heart failure, and ischemia with attendant reperfusion injury share many of the same risk factors, among the most important being oxidative stress and alterations in the blood concentrations of compounds that influence oxidative stress, such as oxidized low-density lipoprotein. In this review, we focus on endothelial cells: cells in the frontline against these disturbances. Because ATP supplies in endothelial cells are relatively independent of mitochondrial oxidative pathways, the mitochondria of endothelial cells have been somewhat neglected. However, they are emerging as agents with diverse roles in modulating the dynamics of intracellular calcium and the generation of reactive oxygen species and nitric oxide. The mitochondria may also constitute critical "targets" of oxidative stress, because survival of endothelial cells can be compromised by opening of the mitochondrial permeability transition pore or by mitochondrial pathways of apoptosis. In addition, evidence suggests that endothelial mitochondria may play a "reconnaissance" role. For example, although the exact mechanism remains obscure, endothelial mitochondria may sense levels of oxygen in the blood and relay this information to cardiac myocytes as well as modulating the vasodilatory response mediated by endothelial nitric oxide.

Superoxide dismutase mimetic tempol inhibits hypoxic pulmonary vasoconstriction in rats independently of nitric oxide production

Hypoxic pulmonary vasoconstriction (HPV), an important physiological mechanism, is regulated by changes in the production of and interactions among reactive oxygen species (ROS). There is controversy, however, over whether HPV is mediated by an increase or a decrease in ROS production. Also, the role of NO in HPV remains unclear. The aim of this study was to investigate whether the inhibition of HPV by the antioxidant tempol was dependent on the concentration of NO, and how its effect was influenced by increased basal pulmonary vascular tone. In isolated rat lungs, we measured vasoconstrictor responses to acute ventilatory hypoxia before and after administration of tempol during perfusion with or without L-NAME. We found that tempol abolished HPV independently of NO production. When we increased basal vascular tone by K(+)-induced depolarization, we also found that tempol completely inhibited HPV. Our results indicate that inhibition of HPV by the superoxide dismutase mimetic tempol does not depend on either NO production or a decrease in basal vascular tone.

ROS generation in endothelial hypoxia and reoxygenation stimulates MAP kinase signaling and kinase-dependent neutrophil recruitment

Reactive oxygen species (ROS)-induced injury has been shown to occur during the reperfusion phase of ischemia-reperfusion and ROS are known to induce signaling events. We hypothesized that oxygen sensing in endothelial cells is also dependent on internal redox changes during hypoxia and that endothelial cells respond to changing oxygen environments via signaling, switching to an inflammatory phenotype. Endothelial cells exposed to relative hypoxia or the mitochondrial inhibitors rotenone, antimycin A, or FCCP show loss of mitochondrial membrane potential. During hypoxia, an increase in cytoplasmic ROS and glutathione S-transferase activity occurred, suggesting changes in intracellular redox state, mimicked with rotenone or FCCP but inhibited by antimycin A. Phosphorylation of stress-responsive mitogen-activated protein kinases occurred in hypoxia and was rapid and prolonged. Phosphorylation was inhibited by vitamin C, N-acetyl cysteine, or antimycin A. Chelation of intracellular calcium inhibits phosphorylation but the mitochondrial transition pore inhibitor cyclosporin A had no effect. Reoxygenation caused a further round of signaling, which was rapid but transient. Functionally, adhesion of neutrophils after hypoxia-reoxygenation under flow is ROS, P-selectin, and MAPK dependent. Therefore, changes in cellular signaling and phenotype are abrogated by ROS scavengers and suggest their use as therapeutic agents in ischemia-reperfusion.

Three distinct mechanisms generate oxygen free radicals in neurons and contribute to cell death during anoxia and reoxygenation

Ischemia is a major cause of brain damage, and patient management is complicated by the paradoxical injury that results from reoxygenation. We have now explored the generation of reactive oxygen species (ROS) in hippocampal and cortical neurons in culture in response to oxygen and glucose deprivation or metabolic inhibition and reoxygenation. Fluorescence microscopy was used to measure the rate of ROS generation using hydroethidine, dicarboxyfluorescein diacetate, or MitoSOX. ROS generation was correlated with changing mitochondrial potential (rhodamine 123), [Ca2+]c (fluo-4, fura-2, or Indo-1), or ATP consumption, indicated by increased [Mg2+]c. We found that three distinct mechanisms contribute to neuronal injury by generating ROS and oxidative stress, each operating at a different stage of ischemia and reperfusion. In response to hypoxia, mitochondria generate an initial burst of ROS, which is curtailed once mitochondria depolarize or prevented by previous depolarization with uncoupler. A second phase of ROS generation that followed after a delay was blocked by the xanthine oxidase (XO) inhibitor oxypurinol. This phase correlated with a rise in [Mg2+]c, suggesting XO activation by accumulating products of ATP consumption. A third phase of ROS generation appeared at reoxygenation. This was blocked by NADPH oxidase inhibitors and was absent in cells from gp91(phox-/-) knock-out mice. It was Ca2+ dependent, suggesting activation by increased [Ca2+]c during anoxia, itself partly attributable to glutamate release. Inhibition of either the NADPH oxidase or XO was significantly neuroprotective. Thus, oxidative stress contributes to cell death over and above the injury attributable to energy deprivation.

Hypoxia reduces KCa channel activity by inducing Ca2+ spark uncoupling in cerebral artery smooth muscle cells

Arterial smooth muscle cell large-conductance Ca(2+)-activated potassium (K(Ca)) channels have been implicated in modulating hypoxic dilation of systemic arteries, although this is controversial. K(Ca) channel activity in arterial smooth muscle cells is controlled by localized intracellular Ca(2+) transients, termed Ca(2+) sparks, but hypoxic regulation of Ca(2+) sparks and K(Ca) channel activation by Ca(2+) sparks has not been investigated. We report here that in voltage-clamped (-40 mV) cerebral artery smooth muscle cells, a reduction in dissolved O(2) partial pressure from 150 to 15 mmHg reversibly decreased Ca(2+) spark-induced transient K(Ca) current frequency and amplitude to 61% and 76% of control, respectively. In contrast, hypoxia did not alter Ca(2+) spark frequency, amplitude, global intracellular Ca(2+) concentration, or sarcoplasmic reticulum Ca(2+) load. Hypoxia reduced transient K(Ca) current frequency by decreasing the percentage of Ca(2+) sparks that activated a transient K(Ca) current from 89% to 63%. Hypoxia reduced transient K(Ca) current amplitude by attenuating the amplitude relationship between Ca(2+) sparks that remained coupled and the evoked transient K(Ca) currents. Consistent with these data, in inside-out patches at -40 mV hypoxia reduced K(Ca) channel apparent Ca(2+) sensitivity and increased the K(d) for Ca(2+) from approximately 17 to 32 microM, but did not alter single-channel amplitude. In summary, data indicate that hypoxia reduces K(Ca) channel apparent Ca(2+) sensitivity via a mechanism that is independent of cytosolic signaling messengers, and this leads to uncoupling of K(Ca) channels from Ca(2+) sparks. Transient K(Ca) current inhibition due to uncoupling would oppose hypoxic cerebrovascular dilation.

Function of Kv1.5 channels and genetic variations of KCNA5 in patients with idiopathic pulmonary arterial hypertension

The pore-forming alpha-subunit, Kv1.5, forms functional voltage-gated K(+) (Kv) channels in human pulmonary artery smooth muscle cells (PASMC) and plays an important role in regulating membrane potential, vascular tone, and PASMC proliferation and apoptosis. Inhibited Kv channel expression and function have been implicated in PASMC from patients with idiopathic pulmonary arterial hypertension (IPAH). Here, we report that overexpression of the Kv1.5 channel gene (KCNA5) in human PASMC and other cell lines produced a 15-pS single channel current and a large whole cell current that was sensitive to 4-aminopyridine. Extracellular application of nicotine, bepridil, correolide, and endothelin-1 (ET-1) all significantly and reversibly reduced the Kv1.5 currents, while nicotine and bepridil also accelerated the inactivation kinetics of the currents. Furthermore, we sequenced KCNA5 from IPAH patients and identified 17 single-nucleotide polymorphisms (SNPs); 7 are novel SNPs. There are 12 SNPs in the upstream 5' region, 2 of which may alter transcription factor binding sites in the promoter, 2 nonsynonymous SNPs in the coding region, 2 SNPs in the 3'-untranslated region, and 1 SNP in the 3'-flanking region. Two SNPs may correlate with the nitric oxide-mediated decrease in pulmonary arterial pressure. Allele frequency of two other SNPs in patients with a history of fenfluramine and phentermine use was significantly different from patients who have never taken the anorexigens. These results suggest that 1) Kv1.5 channels are modulated by various agonists (e.g., nicotine and ET-1); 2) novel SNPs in KCNA5 are present in IPAH patients; and 3) SNPs in the promoter and translated regions of KCNA5 may underlie the altered expression and/or function of Kv1.5 channels in PASMC from IPAH patients.

Hypoxia-induced and stress-specific changes in chromatin structure and function

Cellular adaptation to stress relies on specific, regulated responses to evoke changes in gene expression. Stresses such as hypoxia, heat shock, oxidative stress and DNA-damage activate signaling cascades that ultimately lead to either induction or repression of stress-responsive genes. In this review, we concentrate on the mechanisms by which stress-induced signaling promotes alterations in chromatin structure, whether the read-out is activation or repression of transcription. Specific alterations in chromatin are highly regulated and dictated by the type of imposed stress. Our primary focus is on the types of chromatin alterations that occur under hypoxic conditions, which exist within a majority of tumors, and to compare these to changes in chromatin structure that occur in response to a wide variety of cellular stresses.

Mitochondrial reactive oxygen species-mediated signaling in endothelial cells

Once thought of as toxic by-products of cellular metabolism, reactive oxygen species (ROS) have been implicated in a large variety of cell-signaling processes. Several enzymatic systems contribute to ROS production in vascular endothelial cells, including NA(D)PH oxidase, xanthine oxidase, uncoupled endothelial nitric oxide synthase, and the mitochondrial electron transport chain. The respiratory chain is the major source of ROS in most mammalian cells, but the role of mitochondria-derived ROS in vascular cell signaling has received little attention. A new paradigm has evolved in recent years postulating that, in addition to producing ATP, mitochondria also play a key role in cell signaling and regulate a variety of cellular functions. This review focuses on the emerging role of mitochondrial ROS as signaling molecules in vascular endothelial cells. Specifically, we discuss some recent findings that indicate that mitochondrial ROS regulate vascular endothelial function, focusing on major sites of ROS production in endothelial mitochondria, factors modulating mitochondrial ROS production, the physiological and clinical implications of endothelial mitochondrial ROS, and methodological considerations in the study of mitochondrial contribution to vascular ROS generation.

Classical transient receptor potential channel 6 (TRPC6) is essential for hypoxic pulmonary vasoconstriction and alveolar gas exchange

Regional alveolar hypoxia causes local vasoconstriction in the lung, shifting blood flow from hypoxic to normoxic areas, thereby maintaining gas exchange. This mechanism is known as hypoxic pulmonary vasoconstriction (HPV). Disturbances in HPV can cause life-threatening hypoxemia whereas chronic hypoxia triggers lung vascular remodeling and pulmonary hypertension. The signaling cascade of this vitally important mechanism is still unresolved. Using transient receptor potential channel 6 (TRPC6)-deficient mice, we show that this channel is a key regulator of acute HPV as this regulatory mechanism was absent in TRPC6(-/-) mice whereas the pulmonary vasoconstrictor response to the thromboxane mimetic U46619 was unchanged. Accordingly, induction of regional hypoventilation resulted in severe arterial hypoxemia in TRPC6(-/-) but not in WT mice. This effect was mirrored by a lack of hypoxia-induced cation influx and currents in smooth-muscle cells from precapillary pulmonary arteries (PASMC) of TRPC6(-/-) mice. In both WT and TRPC6(-/-) PASMC hypoxia caused diacylglycerol (DAG) accumulation. DAG seems to exert its action via TRPC6, as DAG kinase inhibition provoked a cation influx only in WT but not in TRPC6(-/-) PASMC. Notably, chronic hypoxia-induced pulmonary hypertension was independent of TRPC6 activity. We conclude that TRPC6 plays a unique and indispensable role in acute hypoxic pulmonary vasoconstriction. Manipulation of TRPC6 function may thus offer a therapeutic strategy for the control of pulmonary hemodynamics and gas exchange.

Thiol oxidation causes pulmonary vasodilation by activating K+ channels and inhibiting store-operated Ca2+ channels

Cellular redox change regulates pulmonary vascular tone by affecting function of membrane and cytoplasmic proteins, enzymes, and second messengers. This study was designed to test the hypothesis that functional modulation of ion channels by thiol oxidation contributes to regulation of excitation-contraction coupling in isolated pulmonary artery (PA) rings. Acute treatment with the thiol oxidant diamide produced a dose-dependent relaxation in PA rings; the IC50 was 335 and 58 microM for 40 mM K+ - and 2 microM phenylephrine-induced PA contraction, respectively. The diamide-mediated pulmonary vasodilation was affected by neither functional removal of endothelium nor 8-bromoguanosine-3'-5'-cyclic monophosphate (50 microM) and HA-1004 (30 microM). A rise in extracellular K+ concentration (from 20 to 80 mM) attenuated the thiol oxidant-induced PA relaxation. Passive store depletion by cyclopiazonic acid (50 microM) and active store depletion by phenylephrine (in the absence of external Ca2+ both induced PA contraction due to capacitative Ca2+ entry. Thiol oxidation by diamide significantly attenuated capacitative Ca2+ entry-induced PA contraction due to active and passive store depletion. The PA rings isolated from left and right PA branches appeared to respond differently to store depletion. Although the active tension induced by passive store depletion was comparable, the active tension induced by active store depletion was 3.5-fold greater in right branches than in left branches. These data indicate that thiol oxidation causes pulmonary vasodilation by activating K+ channels and inhibiting store-operated Ca2+ channels, which subsequently attenuate Ca2+ influx and decrease cytosolic free Ca2+ concentration in pulmonary artery smooth muscle cells. The mechanisms involved in thiol oxidation-mediated pulmonary vasodilation or activation of K+ channels and inhibition of store-operated Ca2+ channels appear to be independent of functional endothelium and of the cGMP-dependent protein kinase pathway.